Alpha-lactalbumin, a modifier protein that changes the substrate specificity of galactosyltransferase, to promote the synthesis of lactose, is found in the mammary glands of lactating mammals and in milk. Molecules similar to mammary gland Alpha-lactalbumin but distinct in their modifie activity have been found in the epididymal fluid. This activity differs from mammary gland Alpha-LA activity in that it transfers galactose from UDP-galactose to either glucose or myoinositol with equal efficiency. The products of these reactions, lactose and galactinol were characterized by paper chromatography. Using rat mammary gland Alpha-lactalbumin cDNA clone as a hybridization probe, RNA sequences homologous to Alpha-lactalbumin mRNA were also detected in total RNA from rat epididymis. This finding suggests that Alpha-lactalbumin or similar molecules, in addition to regulating lactose synthesis in the mammary gland, may have other important functions, e.g., synthesizing specific oligosaccharide sequence on the cell surface glycoproteins which are recognized as new antigenic determinants. Specifically in the male reproductive tract, where lactose is absent and free glucose levels are barely detectable, Alpha-LA-like activity may modulate sperm surface glycoproteins which may plan an important role in sperm-egg surface interactions during fertilization.